This invention relates to strip detectors in general and more particularly to an improved strip detector and a method of making such a detector.
Strip detectors used for measuring ionizing radiation in which electrodes are disposed on both flat sides of a semi-conductor at angles to each other such that each intersection point forms a detector are presently known. For example, such detectors, which are also known as chess-board detectors, are disclosed in Swiss Pat. No. 460,962. The detector disclosed therein contains on a flat top side a large plurality of strip-like electrodes arranges parallel to each other and each of which is covered by a surface barrier layer. The electrodes comprise a vapor deposited gold layer which forms a schottky-type junction with the semi-conductor wafer on which it is deposited. On the opposite side of the semi-conductor wafer a plurality of strip-like electrodes of vapor deposited aluminum of a resistive nature and which are rotated with respect to the gold electrodes by a predetermined angle, preferably 90.degree., are provided. The semi-conductor wafer will generally consist of germanium or silicon. In this arrangement the intersection points of the electrodes on the two flat sides of the semi-conductor body make up a plurality of individual detectors for incident radiation. The arrangement is particularly useful as a detector for charged particles as well as a detector for gamma and x-rays and, in addition for light radiation which has an energy of more than approximately 1.1 electron volts. This arrangement provides a spacial resolution when measuring radiations of these types.
In U.S. application Ser. No. 195,345 a detector arrangement of chess-board design and providing spacial resolution is shown in which gamma radiation measurements and provided as a visible output. The system operates similar to that of a conventional mosaic system with the photo multipliers normally used replaced by the intersection points of the strip electrodes which provide the individual detectors. Thus, rather than use a conventional florescent screen image intensifier, the semi-conductor detector which, when struck by an electron beam, delivers corresponding electrical signals is used. From these signals the center of the electron beam is determined. At the same time the number of electrons can be recorded. Gamma quanta which has generated, at the input of the image intensifiers, a predetermined light distribution in a suitable detector system, for example, a curved single crystal are localized and their energy determined. Because the detector arrangement includes a large plurality of strip shaped contacts the point of incidence of the gamma quanta can be approximately determined in a digital form. The pulse height ratios of adjacent strips furnish in addition an analog correction signal. As applied in certain applications, these detectors of the prior art present problems. For example, if the strip detector is used as a localizing system for determining the center of gravity of the electron distribution of the image intensifier of a gamma camera it must be able to withstand, after installation in the image amplifier, high temperatures which can reach in excess of 300.degree.C during a baking out process. To fulfill its purpose it must of course be able to withstand these temperatures without its electrical property being affected. Generally, the detectors of the prior art having vapor deposited electrodes require plastic parts, particularly araldite layers which will burn up at temperatures exceeding 150.degree. making the detector useless. The barrier layers used with the vapor deposited gold electrons in addition are not suited for high temperatures because at these temperatures the metal will diffuse into the semi-conductor body and destroy the metal-semi-conductor junction which serves as the barrier layer.
In these prior art strip detectors, the aluminum strips are attached to an electrically insulating very thin, intermediate layer of silicone dioxide. The thickness of this layer generally does not exceed approximately 100 angstroms. Thus if the finished detector is treated at a temperature above 200.degree.C, the aluminum can alloy itself through the thin silicon oxide layer and form a metal-semi-conductor contact with the semi-conductor body. If this occurs dE-dx operation is no longer possible.
An additional problem in the prior art detectors, which are also in some cases referred to as counters, is that charged particles or quanta entering the sensitive zone beneath an electrode are not always recorded by the corresponding intersection point on the other side of the semi-conductor, but instead are recorded by an adjacent intersection point. This phenomenon referred to as cross-talk results in a false measuring result.
Another surface boundry layer counter is disclosed in "Striped semi-conductor device 5441" published by A. B. Atomenergi Sect. S.S.I., Studicik Nykoping, Sweden. The detector disclosed therein contains an N conduction silicon semi-conductor body upon which parallel strip-like electrodes of gold with a thickness of about 250 angstroms and a width of about 0.8 mm are deposited. The electrodes are arranged parallel each other with a spacing of about 0.2 mm and each is provided with an electrode lead. Contamination present on the surface of these counters can generate through adsorption, an inversion layer which forms a P induction channel between the electrode strips. This will result in cross-talk on the top side of the device. Although such cross-talk can be prevented, relatively complex means are required for that purpose. Such means are described in U.S. Pat. No. 3,624,399.
In addition to the problems noted above with prior art devices they are not capable of stable operation in a vacuum over extended periods of time. Thus it can be seen that there is a need for an improved device of this nature which can withstand high temperatures, can operate in a vacuum and is not subject to high levels of cross-talk.